This application relates to a field programmable radio frequency communications systems in general, and more particularly to a system and method for trellis coded modulation ("TCM").
Descriptions of the various components of the system are contained in co-pending patent applications owned by the assignee hereof and filed concurrently herewith, specifically: U.S. Pat. No. 6,091,765, entitled "Reconfigurable Radio System Architecture And Method Therefor"; U.S. patent application Ser. No. 09/184,716 entitled "A Control System For Controlling the Processing Data of a First In First Out Memory and Method Therefor"; U.S. patent application Ser. No. 09/184,940 entitled "Configurable Circuits for Field Programmable Radio Frequency Communications Equipment and Methods Therefor"; U.S. patent application Ser. No. 09/184,710 entitled "A System For Accelerating the Reconfiguration of a Transceiver and Method Therefor"; U.S. patent application Ser. No. 09/184,709 entitled "A Field Programmable Radio Frequency Communications Equipment Including A Configurable IF Circuit, And Method Therefore"; U.S. patent application Ser. No. 09/184,711 entitled "A Field Programmable Modulator-Demodulator Arrangement For Radio Frequency Communications Equipment, And Method Therefor"; U.S. patent application Ser. No. 09/184,708 entitled "A Digital Noise Blanker For Communications Systems and Methods Therefor"; U.S. patent application Ser. No. 09/184,712 entitled "TCM Revisiting System and Method"; U.S. patent application Ser. No. 09/184,941 entitled "Least Squares Phase Fit As Frequency Estimate"; U.S. patent application Ser. No. 09/184,715 entitled "Polar Computation of Branch Metrics For TCM"; U.S. patent application Ser. No. 09/184,713 entitled "Receiver For a Reconfigurable Radio System and Method Therefore"; each of which is incorporated herein by reference.
In digital phase modulation, the transmission of data is accomplished by shifting of the carrier phase to a specific value for each symbol transmitted; such modulation usually referred to as phase shift keying ("PSK"). In a PSK system, phase shift points can be considered as being located about a 360 degree constellation of points representing a sine wave. The number of data points (branches) used in a PSK system will depend upon the complexity of data to be transmitted. For example, the PSK system can take the form of a binary PSK with the constellation divided into two data points, a QPSK with the constellation divided into four data points, an 8 PSK with the constellation phase divided into eight branches, a 16 PSK with the constellation divided into 16 branches. The 16 PSK system will include 16 data points about the constellation designated as 0000, 0001, 0010, 0011, etc. As the number of data points (branches) increase, the complexity of the communications system increases.
In these types of systems, the data often involves Trellis Coded Modulation (TCM). A receiver system for receiving the trellis modulated signals often includes a Viterbi algorithm to decode the information. In a given PSK system only a finite number of sets can be transmitted. The Viterbi algorithm functions to help make a determination of the probability that the symbol set received was the symbol set transmitted.
In a Trellis Coded Modulation where each symbol is to transmit k bits, some number m of the least significant bits are sent to an encoder which outputs m+p bits to map into the waveform which has p possible symbols where p=2 ** k+p. The k-m bits are not encoded. The number of possible states that the encoder may have is dependent on the constraint length of the code. In a trellis representation of the waveform, the uncoded bits are represented as parallel paths in the transitions between trellis states.
For optimum demodulation of TCM, the parallel paths are followed through the trellis. This dramatically complicates the decoding of Trellis Codes compared to normal convolutional codes because branch metrics have to be computed for all the parallel paths, and the best parallel path needs to be stored for all surviving paths.
The standard way of reducing the complexity is to make a decision on the MSBs immediately and then decode the LSB in light of that decision. This eliminates the parallel paths and therefore reduces the complexity of the decoding algorithm. FIGS. 1a and 1b show a trellis with parallel paths and a trellis without parallel paths respectively.
While reducing the complexity of the decoding algorithm, making an immediate decision on the MSBs also increases the likelihood of an error for the MSBs because the effect of the LSBs is to shift the waveform nearer to the decision boundaries of the MSBs. This makes the MSB decision more susceptible to noise. FIG. 2 illustrates this concept for an 8 PSK system. It can be seen that the distance of the signal from the MSB decision boundary depends on the values of the LSBs. These LSBs then act as an interferer when deciding the MSBs.
Accordingly, it is an object of the present invention to provide a novel method and system for reducing the likelihood of errors induced by the LSBs in TCM.
It is another object of the present invention to provide a novel system and method for eliminating the parallel paths of a trellis in TCM.
Further, the computation of the trellis branch metrics using textbook approaches are very intensive and require a great deal of microprocessor cycles.
Therefore, it is yet another object of the present invention to provide a novel method and system for processing samples by exploiting the inherent qualities of the form by which they are represented.
In TCM, the decoding of the received signal may be accomplished through the use of Viterbi decoders. Prior art Viterbi decoders exist which improve performance of a convolutional decoder by 1.5 dB. However, such decoders greatly increase the complexity of the algorithm.
It is yet a further object of the present invention to provide a novel method and system to reduce the complexity of the processing associated with Viterbi decoders without reducing the performance of the algorithm.
Finally, when receiving a signal, a frequency estimate must be made on a pure carrier. The effectiveness of the carrier estimate will dictate the required length of the preamble of a data waveform. The shorter the preamble the better for applications such as networking. Processing should be as simple as possible so that the estimate can be done and corrections made before the data starts.
It is still a further object of the present invention to provide a novel method and system for providing frequency estimation of a carrier.
It is further an object of this invention to provide a novel and improved radio frequency receiver that includes a simplified arrangement for providing an estimate of the carrier signal of the received signal and correlate the demodulator to the estimate of the carrier signal.
It is also an object of this invention to provide a novel and improved receiver for receiving and decoding TCM signals.
It is also and object of this invention to provide a novel and improved radio frequency receiver for reducing the likelihood of errors in decoding received TCM signals.
It is also an object of this invention to provide a novel and improved radio frequency receiver for simplified decoding of received TCM signals without significant degradation of performance.
It is also an object of this invention to provide a novel and improved radio frequency receiver for decoding TCM signals involving a decoder arrangement including simplified polar computations and Viterbi decoding.
These and many other objects and advantages of the present invention will be readily apparent to one skilled in the art to which the invention pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.